The shift to bio-based material technologies for not only energy but also chemical refineries, and the conversion of industrial starting materials from petroleum to biomass, have become issues in recent years.
In humans, 3-hydroxybutyric acid (hereinafter also referred to as “3-HB” in the present specification), which is produced from acetyl CoA in the liver, is used as an energy source for the brain when blood glucose levels are low. Further, it can prevent the transfer of enterobacteria into the blood. Therefore, the use of 3-hydroxybutyric acid in infusions, eye-drops, etc., and its use as a starting material for biodegradable plastics have been examined (PTL 1 and PTL 2).
Known processes for producing 3-hydroxybutyric acid are as follows. For example, since this compound is a monomer of poly-3-hydroxybutyrate (hereinafter also referred to as “PHB” in the present specification), PHB is produced by various bacterial cells, and then degraded by lipases, etc., which have been prepared separately, to thereby obtain 3-hydroxybutyric acid, which is a monomer of PHB (PTL 3). NPL 1 shows a process in which 8.7 g/L of 3-hydroxybutyric acid is obtained by using mutants, and NPL 2 shows a process in which 3-hydroxybutyric acid is obtained with a yield of 12 g/L by using a gene recombination technique.
The present inventors examined an efficient process of culturing Spirulina microalgae, and found that a specific basophilic bacterium grew under certain conditions as the only contaminating bacterium. Since the halophilic bacteria generally grew well in a medium with a pH of about 5 to 12 containing a high concentration of sodium, it was presumed that contamination by other bacteria hardly occurred, even under aerobic fermentation. Then, examination of the assimilation of various carbon sources by the halophilic bacteria revealed that a remarkable amount of PHB was accumulated in the cells of the halophilic bacteria (PTL 4).
In the course of subsequent studies, it was found that halophilic bacteria belonging to the genus Halomonas accumulated PHB under aerobic conditions, and that when the aerobic conditions were shifted to microaerobic conditions, 3-hydroxybutyric acid, which is a monomer of PHB, was secreted and produced in the culture medium (PTL 5).
It is known that 3-hydroxybutyric acid is contained in a remarkable amount in breast milk, and is derived from fatty acid, etc., at a low glucose concentration. It is also known that the metabolic speed of 3-hydroxybutyric acid is relatively fast. Methyl esters of 3-hydroxybutyric acid are expected to be useful as health foods, etc., and particularly supplements suitable for athletes.
Thus, 3-hydroxybutyric acid is a very useful starting chemical material as original acidic form mentioned above, or as a polymer comprising 3-hydroxybutyric acid as a constituent unit.